1. Field of the Invention
The present invention relates to method and apparatus for cleaning a substrate in a printing apparatus, and, more particularly, to a method and apparatus for cleaning the substrate by applying variable pressure to the substrate.
2. Discussion of the Related Art
A typical document copier includes an electrostatic printer with a belt having a photoconductive surface. To transfer an image onto a sheet of paper, the printer charges the belt to a uniform potential, and subsequently exposes the belt to a pattern of light corresponding to the image. Parts of the belt exposed to the light are discharged, resulting in an electrostatic latent image being formed on the belt. The portion of the belt having the electrostatic image then passes a development station that deposits toner on the belt in the pattern of the image, resulting in a toner powder image being formed on the belt. A piece of paper is then tacked to the belt and then removed from the belt, resulting in an image being formed on the paper.
In a printing process of this type, some residual toner particles will remain on the photoconductive surface after the toner image has been transferred to the paper. In addition to the residual toner, other residual particles, such as paper debris, additives and plastic, are left behind on the surface after image transfer. The residual particles should be mostly removed prior to the next printing cycle to avoid their interference with production of another image.
Various methods may be used for removing residual particles, such as methods employing a cleaning brush, a cleaning web, or a cleaning blade of a rubber-like material such as polyurethane. Blade cleaning scrapes or wipes across the belt to remove the residual particles from the belt. Blade cleaning is a desirable method for removing residual particles due to its simplicity and economy. Blade cleaning entails frictional contact with the belt, however, which degrades the blade over a period of time.
A conventional blade cleaning method applies a constant blade pressure against the substrate. The constant pressure is chosen based on the many contingencies that might affect cleaning performance. The resulting constant pressure is more than is needed under some conditions, thereby accelerating the wear rate of the blade. If an over-pressured blade is configured in "doctor mode," the blade will be more susceptible to fold-under, resulting in immediate destruction of the blade and perhaps even damage to the belt. More particularly, lesser amounts of residual toner to be cleaned results in increased friction and adhesion between the blade and the belt. The increase in the friction and adhesion cause the blade to be drawn further downstream in the process direction, with an increase, therefore, in the pressure of the blade against the belt. This increase in pressure initiates a positive feedback mechanism which increases the friction and adhesion, which increases the pressure, etc.
Thus, excessive frictional contact results from excessive pressure on the blade, leading to premature blade failure. Insufficient pressure on the blade, however, leads to insufficient cleaning of the substrate.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 3,918,809 to Hwa discloses an apparatus for cleaning liquid developer from an upwardly moving support surface, such as a reusable surface for carrying latent electrostatic images. Cleaning blades clean the support surface. Separate members hold the cleaning blades in contact with the support surface.
U.S. Pat. No. 5,034,774 to Higginson et al. discloses an apparatus for applying toner for developing an electrostatic latent image formed on the charge retaining surface of a moving recording medium. The apparatus includes compliant cleaning blades for contacting a drying roller to prevent agglomeration of papers fibers and toner particles on the interface between the roller and the scraper blade.